The photosensitive drum surface of an image forming apparatus having a photosensitive drum and designed to form an image by electrophotography is required to be electrified so as to show an appropriate (typically uniform) potential distribution in any environment in order to constantly produce images of a stabilized quality. For this reason, a functional device for measuring the electric potential of the photosensitive drum surface by means of a potential measurement apparatus and feedback-controlling the potential of the photosensitive drum surface so as to keep the potential uniform, utilizing the outcome of the measurement, is generally mounted in the image forming apparatus.
Potential measurement apparatus to be used for such applications are required to have a functional feature of measuring the surface potential of an object of measurement without contacting the object. This is because the potential distribution of the photosensitive drum surface becomes non-uniform and a disturbed image can be formed when the potential measurement apparatus touches the photosensitive drum surface.
A technique of minutely fluctuating the capacitance between the object of measurement and the detection electrode arranged vis-à-vis the object of measurement to obtain a current signal having an amplitude proportional to the surface potential of the object of measurement is utilized for the principle of measurement of contactless type potential measurement apparatus.
Now, the principle of potential measurement of contactless type potential measurement apparatus will be described below.
A quantity of electricity Q that is proportional to the surface potential V of the object of measurement is induced on the detection electrode by the electric field generated between the surface of the object of measurement having a certain electric potential and the detection electrode contained in the potential measurement apparatus.
The relationship between Q and V is expressed by formula (1) below:Q=CV  (1)where C is the capacitance between the detection electrode and the surface of the object of measurement. From the formula (1), it will be seen that the surface potential of the object of measurement can be obtained by measuring the quantity of electricity Q induced on the detection electrode.
However, it is actually difficult to directly measure the quantity of electricity Q that is induced on the detection electrode quickly and accurately. For this reason, a technique of detecting the surface potential of the object of measurement by periodically changing the capacitance C between the detection electrode and the object of measurement and observing the AC signal generated at the detection electrode is employed as practical measurement method.
That the above-described technique can provide the surface potential of the object of measurement will be shown below. To express the capacitance C as a function of time t, the AC signal and hence the potential detection signal current i generated at the detection electrode is expressed by formula (2) below on the basis of the formula (1) because the potential detection signal current i is the time differential value of the quantity of electricity Q induced at the detection electrode.i(t)=dQ/dt=d(CV)/dt  (2)
When the changing rate of the surface potential V of the object of measurement is sufficiently small relative to the changing rate of the capacitance C, V can be regarded as constant for the differentiating time dt. Therefore, the formula (2) can be expressed as formula (3) below.i(t)=dQ(t)/dt=V·dC(t)/dt  (3)
From the formula (3), the surface potential of the object of measurement can be obtained by measuring the amplitude of the AC signal because the magnitude of the potential detection signal current i generated at the detection electrode is expressed as a linear function of the surface potential V of the object of measurement. Additionally, from the formula (3), if the changing rate of the capacitance remains same, the magnitude of the AC signal relative to the surface potential of the object of measurement and hence the sensitivity of the potential measurement apparatus is proportional to the changing rate of the capacitance.
As described above, when measuring the electric potential V of the surface of the object of measurement, the magnitude of the capacity C between the detection electrode and the object of measurement is desirably periodically modulated in order to accurately measure the quantity of the electric charge Q that appears on the detection electrode. A technique that can be used to modulate the capacity C is to periodically change the distance x between the detection electrode and the object of measurement. A unit that can be used to periodically change the distance between the detection electrode and the surface of the object of measurement is to arrange the detection electrode at the front end of a vibrating body and vibrate the vibrating body in a direction perpendicular to the surface of the object of measurement (see the embodiment that will be described hereinafter).
The distance x between the detection electrode and the object of measurement is to be periodically changed because the capacitance C between the detection electrode and the surface of the object of measurement illustrates a relationship that is approximately expressed by formula (4) below:C=A·S/x  (4),where A is the constant of proportion specific to the dielectric constant of the substance of the object of measurement, S is the area of the detection electrode and x is the distance between the detection electrode and the surface of the object of measurement. From the formula (4), it will be seen that the capacitance C is changed periodically when the distance x is changed periodically. Note that A and S are constant.
However, a potential measurement apparatus adapted to detect as an electric current the change in the electric charge induced at the detection electrode by electrostatic induction is sensitive to the distance that operates as reference for the object of measurement and the detection electrode (the neutral distance that operates as reference relative to the change) and the distance strongly influences the accuracy of measurement and the resolution. While the distance from the detection electrode to the object of measurement changes periodically, the distance between the object of measurement and the detection electrode when the detection electrode is located at the neutral position that operates as reference is referred to as neutral distance in this specification. Thus, the neutral distance is a distance that needs to be held to a constant value regardless of change of the distance between the detection electrode and the surface of the object of measurement when the vibrating body is actually vibrating. While the neutral distance ideally needs to be held to a constant value, the neutral distance fluctuates in reality due to the vibrations of the image forming apparatus in operation, the eccentric rotation of the photosensitive drum and the change with time or due to heat of the distances among the components of the image forming apparatus. As the neutral distance changes, the measured value of the potential measurement apparatus is shifted from the true value. In short, the measured potential fluctuates depending on the change in the neutral distance.
Therefore, known potential measurement apparatus employ a booster circuit section for suppressing the distance dependency relative to the object of measurement for the purpose of feedback control. With a typical configuration of known potential measurement apparatus, information on the potential of the object of measurement is sensed by means of a sensing circuit including a detection electrode and subsequently the signal is amplified by means of an amplifying circuit. Then, the signal obtained from the amplifying circuit is detected and boosted by way of a detecting circuit and a boosting circuit. The boosted voltage is then used as feedback voltage and applied to the sensing circuit by way of a shield line. Then, the equilibrium point with the potential of the object of measurement is determined and the voltage applied to the sensing circuit is used as the potential of the object of measurement to suppress the distance dependency (see Japanese Patent Application Laid-Open No. 07-244103).